How does lateral pressure of soils affect the drain performance?

The presence of lateral pressure of backfilling sidesoils induces bending
moments in the opposite direction from those produced by vertical loads and bedding support reaction. Such bending moment reduces the flexure in pipe wall and as a result this causes an equivalent increase in supporting strength of the pipe.

The lateral pressure of backfilling soils on drains is affected by the deflection of drains. With no occurrence of deflection, lateral pressure induced is in the form of active pressure. If pipe deflection occurs, the drain increases its horizontal dimension so that passive pressure is developed.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.